1. Technical Field
The present invention relates to broadcasting RF signals and, more particularly, to improvements permitting both analog and digital components to be transmitted employing a common amplifier.
2. Description of the Prior Art
The introduction of digital audio broadcasting (DAB) and other forms of vector modulation require simultaneous amplitude and phase modulation of the RF carrier. Conventional FM broadcast transmitters utilize nonlinear RF power amplifiers that cannot convey the amplitude variations needed to accurately replicate the vector modulation.
In the prior art, it is known to employ separate amplification of the vector modulation signal and of the analog FM signal. An example of this is disclosed in FIG. 1 herein. In this example, the output V2 of an existing FM transmitter 10, illustrated as a main FM transmitter, is combined with the output V3 from a digital transmitter referred to as a digital TX (linear) transmitter. The input to the main transmitter 10 is obtained from a conventional FM signal source 14 while the input to the digital transmitter 12 is obtained from a conventional IBOC source 16. The outputs V2 and V3 of these transmitters are combined in a conventional coupler C, which typically is a 10 dB coupler. The coupler, which is sometimes known as an output coupler, provides an output V1 that is supplied to a transmitting antenna 20 for broadcasting the composite signal. The coupler C has ports 1, 2, 3 and 4 with the voltages V2 and V3 being applied to ports 2 and 3. The output at port 1 is supplied to the antenna 20. Port 4 is coupled to a reject load RL.
Because the outputs V1 and V2 are combined only after they have reached a high level of amplitude (because they have already been amplified by separate amplifiers) this is referred to in the art as “high-level combining” or “separate amplification”. This type of combining results in high losses because the two signals are not correlated. This may be viewed as the penalty paid for the simplicity involved. In a 10 dB coupler, some of the problems noted include the following: the main FM transmitter needs to have enough headroom in order to increase its output power to overcome the combiner insertion loss. This can be very problematic in specific installations without additional headroom to spare. Major hardware upgrade could be necessary to overcome this issue, such as by replacing the existing main FM transmitter with a more powerful transmitter. A second problem with this type of system is that the overall dissipation increases. Besides the power dissipated by digital transmitter 12, additional energy is wasted at this reject load RL where up to 10% of the main transmitter FM output and up to 90% of the output of the digital transmitter will be dissipated. This inefficiency creates additional heat load for the air-conditioning equipment.
Other prior art examples include the U.S. patents to Murphy et al. U.S. Pat. No. 5,315,583 and Papadopoulos et al. U.S. Pat. No. 6,144,705. It will be noted that the example in FIG. 1 requires a separate linear power amplifier to add the vector modulator signal to the existing analog FM signal. This technique, therefore, requires a second transmitter and inefficient RF combining of the two RF signals. Alternatively, the combined analog and digital signals provided in the composite signal, can be amplified together in a single, linear RF amplifier, with low efficiency. This technique is known as linear, common amplification.
It is desired to add the vector modulation amplitude and phase components to the same nonlinear amplifier used to simultaneously amplify the constant amplitude, analog FM signal. This is the subject of the invention herein to be described below. This invention will allow the existing FM broadcast transmitter to add vector modulation to the existing FM signal without the need for a second transmitter and inefficient RF combining equipment.